Why is this rigid-body constraint allowing the object to be partially pushed away?
Heh, I got a little tired of debugging this vehicle of yours, so I remade it.
I stripped out parenting (because the physics engine does not respond to parenting correctly when things have to actuate), and replaced all of it with constraints.

The result:
https://dl.dropboxusercontent.com/u/17630723/Blends/gandalfmobile.blend

As to why the joint is acting weak, it is because it is parented to an object that is 'static' (if things are parented, the physics engine treats them as static), and thus has infinite mass. So by making the target of the constaint into a valid rigid-body (by using a constraint rather than a parent on the tail), this problem was avoided.

Creating constraints using python also provides the ability to control things like the springiness, the angle and the rotation speed of the constraint, things that aren't available through the UI.



On a side note, development of something is better suited to the forum style of reponse as opposed to multiple stack-exchange questions. I would have preffered for the development of this vehicle to be a blenderartists.org thread.